Background: Total hip replacement (THR) is a reliable therapeutic intervention for patients with severe hip involvement. The aim of our study was to determine the long term outcome and associated risk factors of THR in patients with Ankylosing Spondylitis (AS). Materials and Methods: A cross sectional study was performed. Patients ? 18 years diagnosed with Ankylosing Spondylitis (AS) according to modified NY criteria from the ESPAXIA (Estudio de Espondiloartritis Axial IREP Argentina) cohort were included. Demographic, clinical and data related to THR were recorded. Pain, patient global assessment (VAS), disease activity (BASDAI), functional capacity (BASFI), enthesitis (MASES), axial mobility (BASMI) and radiological damage (mSASSS) were assessed. Hips functional capacity was evaluated by the Merle d`Aubigné and Postel method and pelvis x rays were taken to determine: presence of periprosthesic osteolysis of the femoral and acetabular components, fracture, luxation and heterotopic ossification (HO). Results: 190 patients were evaluated and 25 (14,53%) underwent THR. 16 patients were included in the analysis. Nine (56,2%) had bilateral THR with a median time of THR`s evolution of 12,5 years (IQR 8.7-16.7). 25 prosthesis were evaluated. 3 (12%) prosthesis had surgery complications, 3 (12%) had revision surgery. Pain and functional capacity significantly improved after surgery (median VAS for pain previous to surgery was 10 cm (IQR 9.7-10) vs 0 cm (IQR 0-1.2) after surgery). Twenty twopelvis x-rays were taken, 15 prosthetis had heterotopic ossification, 15 femoral osteolysis, 11 acetabularosteolysis, 3 subluxation and 1 peri-prosthesis fracture. Younger age at disease onset was the main variable associated to THR in multivariate analysis [OR 0,91 (95% CI 0,85-0,97) p=0,001]. Conclusion: A substantial relief in pain and improvement of functional capacity were seen in AS patients who underwent THR. Younger age at disease onset was the only predictor associated to THR.